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Erik Selin
Erik Selin

3D artist & all that other stuff

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How to use boolean modifier in Blender

There are many scenarios where the boolean modifier is an excellent solution to modeling problems. In this article we will cover a few of them and how we can make the most out of it in Blender.

To use the boolean modifier, select an object and go to the modifier stack in the properties panel. Press “add modifier” and find boolean. The boolean modifier needs primarily two things to function. An operation and a target object. You can choose these in the modifier interface.

Let’s now widen our understanding of this tool in Blender.

What is a boolean modifier

In Blender, a boolean modifier is a tool that allow us to perform the boolean operations on our 3D geometry. It takes two overlapping objects to make a boolean and depending on the operation, different results will be achieved. There are three different types of boolean operators.

  • Difference
  • Union
  • Intersect

A difference operation means that we take one object and cut out the volume that it shares with another object, essentially “cutting” or “carving” into it based on the shape of the first object.

A union operation will take both objects and try to fuse them together where they intersect. Essentially adding the volumes together and create one object.

The intersect operation will take the two objects and keep only the volume that they both occupy, removing everything else.

How to enable the boolean modifier in blender

In Blender, a boolean is implemented as a modifier. You can reach the modifier stack by going to the wrench tab in the properties panel.

If you want to learn more about the modifier stack, you can read my beginners guide to it here:

Related content: How modifiers work in Blender, an overview

In the modifier stack, press add modifier and choose the boolean modifier. You will notice that the modifiers name is red.

This means that the modifier is currently not doing anything. It is essentially disabled. It needs to know what other object we want to use to complete the boolean operation with.

To enable the modifier, press the eyedropper icon next to the object slot. Then click on the object you want to perform the operation with either in the outliner or in the 3D viewport.

For a more in depth guide on the outliner, you can check out this article:

Related content: How to work with collections(layers) in Blenders outliner

How to cut a shape out of another shape in blender

The most common use case of a boolean in Blender is to cut out a shape from another shape. We do this with the difference operation. Here are the steps:

  • Make sure you are in Object mode
  • Select the object you want to cut into
  • Add a boolean modifier from the modifier tab in the properties panel.
  • Make sure that the operation is set to Difference
  • Press the eyedropper icon in the boolean modifiers interface
  • Press the object you want to cut with.

The objects need to overlap to have an effect. Also, note that the object you select to cut with will be kept intact. Any changes will only apply to the object that has the boolean modifier.

Blenders boolean is not cutting

It can be hard to see the effect at first because the object we used to cut with will still occupy the cutout portion after a difference operation. Therefore, select the object and hit H to hide it.

You can later unhide all objects with Alt+H.

You can also go to the object properties in the properties panel. That is the yellow icon.

Here you can find the viewport display section and change the Display as setting to wire or bounds. This way Blender will only draw this object as a bounding box or wireframe indicating to us that the object is there, but we can still see the effect.

If you experience additional problems I suggest that you read my troubleshooting guide for booleans.

Related content: Boolean modifier problems and how to solve them

It is quite common to run into problems when dealing with booleans, at least for beginner when the topology might not be 100% correct. Blenders boolean operation is quite picky about this.

Boolean union multiple objects

Another common scenario is when we have multiple object that we want to union together to form a single larger object.

We can do this by stacking multiple boolean modifiers on top of each other to union each object into the first one.

Make sure that we add all modifiers to the same object, even if the portion of the mesh overlapping isn’t overlapping with the original object. Here are the steps:

  • Make sure you are in object mode.
  • Select the object that will hold the modifier stack.
  • Add a boolean modifier to the stack.
  • Change the operation to union
  • Press the eyedropper and click the object to use with the boolean modifier.
  • Repeat the last three steps for each object you want to union.
  • Hide all objects that do not have the modifier stack to avoid viewport artifacts due to overlapping geometry.

If you want to continue to manipulate the raw geometry in edit mode once the objects has been merged, press apply on all modifiers.

Boolean intersect

Intersect is the operation I come across the least. However, I still find it valuable from time to time. We can use it to limit the area an object can be in and create broader shapes with it.

The trick here is to think the opposite way of difference. In most cases we want to include most of our object inside an intersect operation. We can also use an intersection to kind of mold an object.

I will not repeat the steps here. By now you know how boolean works. But when we use the intersect operation, whatever is inside the object we select for the boolean modifier will be left over.

A few scenarios where this can be useful is if we want to contain an object in an area of our scene. Perhaps we model for 3D printing, and we don’t want to model outside the area we can print.

Another scenario is that we want to partially mold our object. In this case we may have an inverted shape we want to fit our object inside.

Blender boolean modifier failed to set value

If you see a message in the information bar at the bottom of Blenders interface saying failed to set value as you try to use the eyedropper tool. It means that the object you are trying to input is the same object that has the boolean modifier.

It may be the case that you have two separate meshes, but they are contained within the same object. If so, you can press tab to go into edit mode.

Then select one face, edge or vertex of the mesh you are trying to use as with the boolean modifier. Press CTRL+L to select all connected geometry.

Then press P and choose selection to separate this mesh to become its own object.

We can now tab out to object mode and try to use the eyedropper tool again.

Final thoughts

There are many creative ways we can use the boolean modifier in Blender. But they also divide 3D artists. Some artists say that they destroy your topology while others rely on Boolean on a daily basis.

The trick is to know when they are the right tool and how we can mitigate potential problems. If you want to learn more about how we can troubleshoot and avoid boolean problems you are welcome to read my boolean troubleshooting guide here:

Related content: Boolean modifier problems and how to solve them

This has been more of an introduction to Boolean in Blender and I hope you feel that you have a solid basic understanding.

Thanks for your time.

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